Ophthalmic plastic lenses have conventionally been produced by casting diethylene glycol bis(allyl carbonate) (DEGBAC) monomer or methyl methacrylate (MMA) monomer into a mold or the like and polymerizing one of these monomers. These plastic lenses, as compared with conventional inorganic glass lenses (crown glass lenses), are superior in lightweightness, safety and tintability. These plastic lenses, however, have a refractive index (n.sub.D) of about 1.50 which is lower than the refractive index (n.sub.D) of conventional inorganic glass lenses, i.e. 1.523; accordingly, the plastic lenses have a drawback in that their center thickness and edge thickness must be made large. In order to eliminate such a drawback, it is desired to develop a plastic lens of high refractive index.
As a method for increasing the refractive index of plastic lens, there is known a method comprising copolymerizing a monomer mixture obtained by adding to a conventional monomer (e.g. DEGBAC), another monomer which allows the resulting polymer to have a high refractive index. The high refractive index plastic lens thus obtained is required to not only have a high refractive index but also have good physical and chemical properties as an ophthalmic lens and further good moldability. As a plastic lens satisfying these requirements, there is mentioned a plastic lens consisting of a terpolymer obtained by polymerizing diethylene glycol bis(allyl carbonate) (DEGBAC), diallyl phthalate (DAP) and benzyl metahcrylate (BzMA), described in, for example, Japanese Patent Application Kokai (Laid-Open) No. 212401/1982. Said plastic lens is obtained by casting a monomer mixture of DEGBAC, DAP and BzMA into a mold assembly constituted by glass or metal upper and lower molds and a resin gasket, polymerizing and curing the mixture by means of heating, ultraviolet irradiation or the like, and releasing the resulting polymer from the molds.
The copolymer produced according to the above prior art method has a high refractive index (n.sub.D =about 1.55), good physical and chemical properties and good moldability. However, in its production, when the monomer mixture is casted into the mold assembly constituted by upper and lower molds and a gasket, the monomer mixture easily adheres to the outer surfaces of the molds; if the heat curing is effected with the outer surfaces left in such a state, the monomer mixture adhering to the molds is heated and resultantly the mold outer surfaces become stained with a half-cured viscous liquid adhering thereto. If, under this condition of the stained outer surfaces of the molds, there is effected an operation for releasing the formed plastic lens from the molds, there occurs the adhesion of said viscous liquid to the plastic lens, when the plastic lens is contacted with the mold outer surfaces, thereby causing the staining and, cloudiness and of the lens.
In order to solve these problems, there is currently being adopted a method comprising cleaning stained molds with, for example, a methylene chloride-strong alkali mixed cleaning solution and then releasing a plastic lens from the molds.
However, the DEGBAC-DAP-BzMA copolymer has weak adhesion to the molds even when no releasing agent such as silicon, fluorine or other type releasing agent is used, and therefore the copolymer has the following drawback. That is, when the mold assembly constituted by upper and lower molds and a gasket is disassembled to remove only the gasket and the molds and the copolymer existing therebetween are immersed in the above mentioned methylene chloride-strong alkali mixed cleaning solution, separation occurs between the molds and the copolymer, whereby the cleaning solution penetrates into the molds, easily inviting the surface roughening, and cloudiness and of the plastic lens caused by the above mentioned cleaning solution.
Hence, the object of the present invention is to provide a high refractive index plastic lens which has improved adhesion to molds, consequently causes no separation from the molds even when immersed in the above mentioned cleaning solution and invites no surface roughening, and no cloudiness.